If they want to create animosity between Sora and Xehanort I think Aqua and Ven should tell Sora about all the terrible things Xehanort did to them a decade ago(killing their master and nearly killing Ven). That would get Sora's blood boiling, especially since he could do the same to the ones he cares about unless he is stopped.
Alternatively Xehanort could just kidnap Kairi, but then she'd be a DID for a third time.
Alternatively Xehanort could just kidnap Kairi, but then she'd be a DID for a third time.